1. Field of the Invention
The present invention relates to a composite wheel for a vehicle and, in particular, to a composite wheel wherein a bead seat band, a side ring holding a tire, and a lock ring fixing the bead seat band are provided around a periphery of a rim.
2. Description of the Prior Art
Various structures of a composite wheel for a vehicle have been proposed. For example, JP-B-46-18643 discloses a typical structure of composite wheel. FIG. 7 illustrates a part of the composite wheel disclosed in that patent publication.
Referring to FIG. 7, rim 1 is connected to disk 2 and one end portion of the rim in the axial direction of the wheel is formed as a rim base portion 3. Bead seat band 5 is mounted about the periphery of rim base portion 3 and an O ring 4 is disposed therebetween. Bead seat band 5 is fixed on rim base portion 3 by lock ring 11 via contact surface 10 formed between the bead seat band and the lock ring. A side ring 7 engages flange portion 6 of bead seat band 5 and one of the bead portions 9 of tire 8 is held by the periphery of bead seat band 5 and the inside surface of side ring 7.
As illustrated in FIG. 7, in the conventional composite wheel, an angle K defined between the contact surface 10 and an axis of wheel is about 45.degree. in almost all cases. The reason for setting the angle K to 45.degree. is presumably because a 45.degree. angle would theoretically be the best for performing the necessary functions of preventing bead seat band 5 from moving in the axial direction of the wheel when the tire 8 expands, ensuring the strength of the bead seat band, ensuring the positioning accuracy of the bead seat band on rim base portion 3 in the axial direction of the wheel, and ensuring the strength of lock ring 11.
Applicants have found however, that an angle K of 45.degree. is not always the best for proper wheel performance when it receives the load from the tire, and in fact there may be some problems depending on the angle.
With the structure shown in FIG. 7, the following problems can be supposed. Here, the following symbols are used:
W1; a load due essentially to the air pressure of tire 8 from the tire toward side ring 7, PA1 W2; a load due essentially to the vertical load of tire 8 from the tire toward bead seat band 5, PA1 f; a force slipping bead seat band 5 generated by the load W2, PA1 .mu.; a friction coefficient on contact surface 10.
F; a frictional force at contact surface 10 between bead seat band 5 and lock ring 11 generated by the load W1,
The forces F and f are expressed as follows. Since bead seat band 5 has a ring structure, an wedge effect acts between the bead seat band and lock ring 11. The frictional force F due to the load W1 generated by the air pressure of tire 8 is expressed as: EQU F=.mu..W1/sin K (1)
The force f slipping bead seat band 5 due to the load W2 is expressed as: EQU f=W2.sin K (2)
Now, problems in actual use are considered. If the load W2 exceeds a certain limit value by an increase in the vertical load of tire 8, the force f becomes greater than the force F, thereby causing a slip between bead seat band 5 and lock ring 11 at contact surface 10. Namely, where the slope angle of contact surface 10 is 45.degree., when the load from tire 8 increases, bead seat band 5 slips on lock ring 11 in the direction of arrow A shown in FIG. 7. When the slip occurs, gap B between rim base portion 3 and bead seat band 5, which is located on the side of the load from tire 8, decreases. At the same time, because bead seat band 5 is a ring-shaped structure, the gap B increases on the opposite side of the rim/bead seat band structure. The increase of gap B causes the sealing force of 0 ring 4 to decrease, whereby a leakage of air from tire 8 is likely to occur. In addition, where the slip occurs, the contact state on contact surface 10 changes from a surface contact to a line contact, and, accordingly, a frictional force on the contact surface in the circumferential direction of the wheel greatly decreases. As a result, in accordance with a torque transmission between tire 8 and disk 2, a condition of slip is satisfied at the portion of contact surface 10.
If slip in the circumferential direction occurs, bead seat band 5 moves on rim base portion 3 in the circumferential direction, particularly where a large torque is generated by the acceleration or braking of the vehicle. Such a movement is likely to cause a locking key provided for preventing such a mutual whirl to break. In a tube type tire, then, the tube will be pulled in the circumferential direction and will crack, thereby causing leakage of air from the tire. Similarly, in a tubeless type tire, since the interference of 0 ring 4 becomes nonuniform in the circumferential direction, a leakage of air from the tire is also likely to occur.